I also have an impression that Zigbee et al are more friendly to extremely low-power, battery-operated sensors participating in the network in situations where a WiFi radio might drain down quickly.
You do mention occupancy sensors, though—if you have experience with battery-powered models that work reliably on WiFi, I’d be open to changing my mind.
That makes sense. All of my "smart" devices are wired to power because I don't want to maintain batteries, and "power is plentiful" as you said. But I can see why WiFi would be a detriment for battery-powered devices, and why some devices would be annoying to hard wire to power (door/window sensors come to mind).
> if you have experience with battery-powered models that work reliably on WiFi
I don't. Most of my sensor devices are just generic sensor components wired into a ESP8266 breakout board, plugged into power. Not much that's ready off-the-shelf.
* If you use universal hub like Home Assistant, they are pretty interoperable between various manufacturers
* Devices don't have direct connection to internet (again esp. with HA), so better privacy, they are faster (no cloud lag) and do not depend on internet connection
* Battery life is way better for small devices
* Mesh is nice when you have bigger area to cover
* If you have to use shitty ISP router, it will have issues with large number of devices
* Usually easy push-to-pair setup
And there isn't many downsides - one time cost of some kind of coordinator and very slightly pricier equipment.
Zigbee also has functional mesh features that wifi doesn't. One is designed for high bandwidth single point communication while the other is designed for low bandwidth long range.
AFAIK a big benefit of Zigbee is that it's designed to be low-power. I have motion sensors that last for 2-3 years on a coin battery, depending on location/traffic. Mains-powered devices like lightbulbs act as repeaters in a Zigbee network, so placement can be anywhere.
All of my lightbulbs, occupancy sensors, etc just connect directly to WiFi, and run custom firmware that I wrote so I know exactly what they're doing and how to control them. They make no attempt to access the wider Internet, but they're all on a vlan without Internet access anyway.
It feels like introducing Zigbee to this would just be an extra hub device taking up space, acting as an extra point of failure, and making it more complicated to develop against my devices. As it stands now I can easily manually control devices by piping crap into netcat if I need to for some reason, since they're all just normal IP networked devices. I think I would have to jump though extra hoops to do similar things with Zigbee.
Is the main aspect driving people to Zigbee just that off the shelf consumer smart devices that use WiFi tend to be annoying dogshit, and Zigbee keeps manufacturers in line better? I don't see any reliability or simplicity benefits to it, just the market poisoning WiFi and Zigbee being the only worthwhile alternative.